1. Field of the Invention
This invention relates to a metal mold for so-called sandwich molding in which a core layer resin material is used for the central layer of a molded article and a skin layer resin material is used for the surface layer of the molded article and the core layer resin is molded into the form of a sandwich with the skin layer resin.
2. Related Background Art
The sandwich molding according to the present invention is characterized in physical strength and improved appearance and slidability of the outer surface, and is suitable for the resin molding of the barrel member of a lens barrel.
As the construction of a lens barrel, there is popularly known a construction which has a lens, a lens holding frame, a fixed cylinder, an intermediate cylinder, an operating ring, etc., and in which the operating ring is rotated about the optical axis to thereby move and control the lens holding frame in a direction parallel with the optical axis through the intermediate cylinder, there is known a helicoid cylinder in which helicoid portions are provided on the inner and outer peripheries of the intermediate cylinder to convert the rotational force of the operating ring about the optical axis into the movement of the lens holding frame in a direction parallel with the optical axis. In the case of a lens barrel, the adoption of a metal such as aluminum (Al) or brass (Bs) as the material thereof leads to physical advantages such as excellent strength and high dimensional accuracy, while on the other hand, it poses numerous problems such as heavy weight, high cost resulting from machining of the metal, and limited productivity, and injection molding which uses resin material in place of metal materials has heretofore been practiced. As a material suitable for the helicoid cylinder, there is known polycarbonate, ABS, polybutylene terephthalate (PBT), modified polyphenylene oxide or polyacetal, and further, use has been made of resin materials having glass fiber or carbon fiber mixed therewith to increase mechanical strength.
The polycarbonate material having glass fiber mixed therewith, as compared with the conventional materials, can provide a helicoid cylinder guaranteed in reduced cost, high accuracy and mechanical strength, but it leaves a problem that the glass fiber is exposed on the inner and outer peripheral surfaces of the helicoid cylinder, whereby an improvement is required in the slidability with which the helicoid cylinder is slidably fitted to the inner and outer cylinders thereof.
A solution of the above-noted problem can be achieved by making the cylinder member into a multilayer structure. For example, by the use of the sandwich molding (hereinafter referred to as the SW molding) as described in Japanese Patent Publication No. 50-28464, U.S. Pat. No. 3,809,519, U.S. Pat. No. 3,599,290, U.S. Pat. No. 3,733,156, U.S. Pat. No. 3,801,684, U.S. Pat. No. 3,690,797 and U.S. Pat. No. 3,873,656, a resin material of excellent slidability having no glass fiber mixed therewith for forming the surface layer of a helicoid cylinder is first poured into the cavity of a metal mold for molding the helicoid cylinder, and then a resin material having glass fiber mixed therewith for forming the core of the helicoid cylinder is poured into the cavity, whereby the surface layer is formed of the smooth resin and the core is formed of the resin of great mechanical strength and thus, a helicoid cylinder which solves the aforementioned problem is provided.
In the sandwich molding, there are the following problems remaining to be solved.
(1) A first problem is the eccentricity of the injected amount of core layer resin material.
This problem will hereinafter be discussed in detail.
FIGS. 1A and 1B of the accompanying drawings are cross-sectional views showing the essential portions of a molded article 1 formed by the sandwich molding of the helicoid cylinder in the afore-described lens barrel and each runner 2. The central portion of the molded article indicated by hatching shows the core layer resin portion, and the outer side of the core layer resin portion (the outer surface portion of the molded article which is shown in white) is the skin layer resin portion.
In FIG. 1A, reference character 1a designates the cylinder portion of the helicoid cylinder 1, and reference character 1b denotes the helicoid portion of the helicoid cylinder.
The helicoid portion is fitted between the fixed cylinder and the lens holding cylinder in a lens barrel, is rotated about the optical axis by the operating force of an operating member to move the lens holding cylinder in the direction of the optical axis, thereby accomplishing the focusing operation or the zooming operation.
A molding machine shown in FIG. 2A of the accompanying drawings is used to sandwich-mold the above-described helicoid cylinder.
Skin layer resin is injected from a first injection cylinder 1A and core layer resin is injected from a second injection cylinder 1B, and the skin layer resin A is injected through a sprue 2a for the skin, a runner 2b for the skin and a common runner 2c and further through a gate 2d into the cavity C of a metal mold.
The core layer resin B is injected through a sprue 2e for core, the common runner 2c and the gate 2d into the cavity C.
A predetermined amount of skin layer resin is first injected into the cavity, and then the core layer resin is injected into the cavity, and when the core layer resin comes to the junction 2f of the runner 2b for the skin and the sprue 2e for core, part of the core layer resin B enters the runner 2b for the skin because the injection pressure of the core layer resin is higher than the injection pressure of the skin layer resin. By part of the core layer resin B entering the runner 2b for the skin, the center of the flow axis of the core layer resin B which flows through the runner 2c becomes eccentric (n) from the center P.sub.1 of the runner 2c toward a point P.sub.2 (toward the 2b side), as shown in FIG. 1B, and further, the resin B flows through the gate 2d and thus, the core layer resin in the portion of the cavity indicated at C in FIG. 1A becomes deficient.
As a result, part of the fore end of the helicoid cylinder 1 is not sandwich-molded, but there is created a portion which is weak in strength as compared with the other sandwich-molded portion.
(2) A second problem left to be solved in sandwich molding is a problem that the core layer resin material mixes with the skin layer resin material. This problem will hereinafter be discussed in detail.
For example, when sandwich molding is to be effected by an injection molding machine shown in FIG. 3A of the accompanying drawings to mold a barrel member, the skin layer resin material from a first injection cylinder 4A is injected through the sprue 6a of a fixed plate 6 and through the runners 8a and 8b of a runner plate 8 into the cavity 10A of a metal mold 10. After a predetermined amount of skin layer resin material A has been injected, a core layer resin material B is injected from a second injection cylinder 4B through a sprue 6b and the runners 8a and 8b into the cavity 10A and the core layer resin material advances while pushing forward the previously injected skin layer resin material, whereby sandwich molding is accomplished, but as shown in FIG. 3B of the accompanying drawings, the later injected core layer resin material also enters a runner 8c through which the skin layer resin material passes.
As a result, where molding is to be effected subsequently to the first injection process, if injection is effected from the state of FIG. 3B, the core layer resin B will mix in the sprue 6a through which the skin layer resin A passes and therefore, if the metal mold is then heated and the skin layer resin material and the core layer resin material are successively injected, the skin layer resin and the core layer resin will mix with each other to make sandwich molding impossible.
(3) A third problem in sandwich molding is the non-uniformity of the core layer resin material which enters each runner of a metal mold.
When the above-described barrel member is sandwich-molded by a molding machine shown in FIGS. 4A and 4B of the accompanying drawings, the filling state of resins forming the sandwich becomes an undesirable state.
That is, in the molding machine of FIG. 4A, injection cylinder units 4A and 4B for injecting skin layer resin A and core layer resin B, respectively, are connected to the sprues 6a and 6b of a fixed plate 6, and the resin which have passed through the sprues 6a and 6b are injected through the runners 8a.sub.1, 8a.sub.2, . . . , 8b.sub.1, 8b.sub.2 of a runner plate 8 into the cavity 10A of a metal mold 10.
When the skin layer resin material A is earlier poured into the runners 8a.sub.1, 8a.sub.2, . . . through the sprue 6a by the use of the molding machine of the described construction, the amounts a.sub.2, a.sub.3 and a.sub.4 of skin layer resin material A flowing into the respective runners become different as shown in FIG. 5 of the accompanying drawings.
In FIG. 4A, the skin layer resin material poured in from a skin layer material pouring runner 8a.sub.1 is poured into runners 8a.sub.2, 8a.sub.3 and 8a.sub.4 from the center points thereof, and as regards the amounts of skin layer resin material which enter the respective runners, the skin layer resin material begins to fill the runners 8a.sub.2 and 8a.sub.4 nearest to the runner 8a.sub.1 and the amount of skin layer resin material filling the runner 8a.sub.3 most distant from the runner 8a.sub.1 is small.
If the amounts of skin layer resin material which enter the respective runners differ from one another as shown in FIG. 5, non-uniformity will occur to the sandwich thickness of the skin layer and core layer depending on the position of the cylinder portion of the barrel member, and this will adversely affect the strength and the lubricity of the surface.